In the event that I see my JG coming up top to gank: While I do my best to watch the Mini-map, and I'm sure everyone else can attest to this as well, but sometimes circumstances force you to focus your attention on your IMMEDIATE surroundings; such as if you are low and your enemy laner is making motions as though they are going to dive you. In that event I will ping my HP and MP/Secondary Resource (as whatever the case may be with my chosen character) to HOPEFULLY get my JG to see that I might not be in a prime position to aid them, and their efforts would probably be best used elsewhere.
If they go in regardless, that's their choice. I've made mine, I'm staying where I am and thus are at least preventing them from getting a double-kill in the event that my JG ends up dying in the fight.
If my JG succeeds, then Kudos to them.
Now, granted, that's only an example of ONE possible situation in a game of countless possible scenarios. Here's a different one:
Let's say I'm in a good state of whatever champion I'm playing. Oh we'll say, 50 to 70% HP, and about the same for my MP or other secondary resource. Enough for at least 1 full rotation of abilities, maybe 2, and a few more uses depending on the ones I choose to use. The JG's heading my way, and they ping to alert me to it. So I do my best to get myself into a position to aid them when they arrive. But the enemy laner, perhaps though wards, or simply a "I've got a bad feeling about this" vibe, realize what's being set up for them. Therefore, they turn tail to retreat to the relative safety of their tower as the JG and I try to converge on them. Dependent on my current location when my JG arrives and my chosen champ, I might not be able to stop the enemy laner, and they make it alive to their tower. But the JG, instead of cutting their losses, decides to tower dive and ends up dying before I can even get there to aid them. At that point, I'd just turn around and get myself into a safe position to back.